using System;
using System.Linq;
using System.Threading.Tasks;

namespace AsynchronousEvent
{
    public delegate Task AsyncEventHandler<in TEventArgs>(object sender, TEventArgs args);

    public sealed class AsyncEvent<TEventArgs>
    {
        private readonly object _lock;

        public AsyncEvent()
        {
            _lock = new object();
        }

        private event AsyncEventHandler<TEventArgs> Handler;

        public void Subscribe(AsyncEventHandler<TEventArgs> handler)
        {
            if (handler == null)
            {
                throw new ArgumentNullException(nameof(handler));
            }

            lock (_lock)
            {
                Handler += handler;
            }
        }

        public void Unsubscribe(AsyncEventHandler<TEventArgs> handler)
        {
            if (handler == null)
            {
                throw new ArgumentNullException(nameof(handler));
            }

            lock (_lock)
            {
                Handler -= handler;
            }
        }

        public void RemoveSubscribers()
        {
            lock (_lock)
            {
                Handler = null;
            }
        }

        public Task InvokeAsync(object sender, TEventArgs args)
        {
            if (Handler == null)
            {
                return Task.CompletedTask;
            }

            var tasks = Handler
                        .GetInvocationList()
                        .Cast<AsyncEventHandler<TEventArgs>>()
                        .Select(s => s.Invoke(sender, args));

            return Task.WhenAll(tasks);
        }
    }
}